This invention pertains to a method and apparatus for producing hollow shell foundry cores from granular material by means of a heatless process which employs a catalytic gas.
Many processes have been developed for producing foundry cores including processes to form solid cores and shell cores. Since solid cores require more material and are heavier and more difficult to handle it is desired to produce hollow shell cores which have the advantage of being lighter in weight than solid cores and also require less material, and are therefore more economical to produce.
A process known as the Kroning process was developed during World War II whereby a thermosetting resin is utilized as a binder for granular material such as sand in producing hollow shell cores. The sand granules are precoated with the resin which contains a catalyst and lubricants. This process has a serious drawback due to the fact that the resin utilized hardens only with the application of heat. This is undesirable due to the large amount of fuel required in the production of hollow shell cores. With the increase in the cost of energy and fuels, this process has become uneconomical. Additionally, the core boxes employed with the Kroning process need to be of suitable construction in order to be able to withstand the high temperatures employed in the process. A further disadvantage of the Kroning process is the need for long curing times in the production of shell cores. It is therefore desired to provide a process for manufacturing hollow shell foundry cores wherein no heat is required and wherein the cycle time for the production of cores is relatively short.
Processes have been developed for producing shell cores with a heatless process wherein a pressure differential is created within a mold box or a gassing chamber by the introduction of catalyst gas after shaping the bindercoated sand in a vented pattern. One such process is disclosed in U.S. Pat. No. 4,089,363. The pressure differential generated is required in order to cause the catalyst gas to penetrate the shaped sand mass to the desired depth to create a shell of sufficient thickness. The disadvantage of these processes wherein such pressure differentials are utilized is that they are relatively time consuming and require costly equipment. Another disadvantage of this type of process is that air bubbles may be trapped in the outer layer of the sand mass, thereby causing weak areas in the shell. It is therefore desired to provide a heatless process for producing hollow shell foundry cores having a uniform wall thickness.
A yet further process has been developed for forming hollow shell foundry cores without the addition of heat by means of binder coated sand and a catalyst gas wherein special porous patterns are provided so that gas can flow through the pores of the pattern walls. One patent disclosing such a process is U.S. Pat. No. 4,291,740. In this process a core box is sealed after a sand and binder mixture has been introduced into the pattern and a catalyst gas is then applied through an inlet port in the core box wall. The gas is applied at greater than atmospheric pressure and will therefore flow through the porous walls of the pattern and penetrate into the sand and binder mixture within the pattern. The gas will exert a uniform pressure on the entire surface area of the shaped sand and binder mass enclosed in the pattern. The catalyst gas, due to its being pressurized, will penetrate a distance into the sand and binder material determined by the relative gas pressures. Equilibrium will be achieved between the applied catalyst gas at high pressure and the resisting pressure developed by the trapped air enclosed by the sand and binder mixture. During penetration and diffusion of the catalyst gas through the outer layer of the sand and binder mixture, hardening or curing thereof is effectuated and a hollow core is produced with a wall thickness determined by the extent of catalyst gas penetration. The process of diffusion and penetration of the catalyst gas through the outer layer of the sand and binder mixture is relatively slow since the process is static and time is required for equilibrium to be reached and for the catalyst molecules to penetrate the sand and binder layer. Therefore, the cycle time required to produce a hollow shell core is relative long, which is undesirable. Another undesirable aspect of this process is the fact that the entire pattern must be porous for the catalyst gas to reach the entire outer layer of the sand and binder mixture contained in the pattern. Such patterns are relatively expensive to manufacture. It is therefore desired to provide a method and apparatus for manufacturing hollow shell cores with reduced cycle time and patterns which are less expensive to manufacture.
U.S. Pat. No. 4,311,184 discloses a type of pattern to be used with the porous pattern process as disclosed in U.S. Pat. No. 4,291,740. An alternative pattern is also disclosed wherein apertures are formed in the pattern walls and porous metal elements are inserted in these apertures. During the formation of hollow shell cores in a process utilizing this type of pattern the gas penetrates these porous inserts under superatmospheric pressure of the catalyst gas and reacts with the binder coated sand in the pattern. As explained hereinabove, the process by which the catalyst gas permeates the sand and binder mass in the pattern is by a static diffusion process. As described in both this patent and the '740 patent, equilibrium must be achieved between the incoming pressurized gas and the air trapped in the sand and binder mixture. Thus, the relatively slow diffusion process of the catalyst molecules into the outer layer of the sand and binder mixture occurs after the static conditions have been established between the catalyst gas and the air trapped in the sand and binder mixture. Additionally, since many catalyst molecules will be stripped from the carrier immediately after the catalyst gas contacts the outermost layer of the sand and binder mixture, substantial time is required for additional molecules to travel beyond this outermost layer and penetrate further into the sand and binder mixture contained in the pattern. A further disadvantage of this process is that only the areas of the sand and binder mass immediately surrounding each aperture will have sufficient gas diffused therethrough to completely react with the binder coated sand. Thus, the shell will in effect be formed of hardened lumps of material which are interconnected with thinner areas. This results in an uneven thickness of the hollow shell foundry cores produced by this process. If it were desired to strengthen the shell the curing step could be made longer for more and deeper penetration of the gas into the sand and binder mixture. This is undesirable since the cycle time would thereby be increased. Alternatively, the catalytic gas pressure could be increased, which is also undesirable as it would require equipment which can withstand the higher pressures.
Another embodiment disclosed in U.S. Pat. No. 4,311,184 discloses a further step in the production of hollow shell foundry cores. A continuous flow of gas is established between an inlet and an outlet so that catalyst gas is directed along the pattern section. The continuous flow allows molecular diffusion between the gas and the air enclosed by the sand mass to cause hardening of the areas of the binder coated sand immediately contiguous to the working surface of the pattern section. In this step also, the process for the penetration of the catalyst molecules into the layer of the sand and binder mixture is by diffusion, which, as explained hereinbefore, is by its very nature a slow process. It is therefore desired to provide a process and apparatus for the heatless production of hollow shell foundry cores which is relatively fast and forms a hollow foundry core shell which has a uniform wall thickness of adequate strength.